Conventionally internal combustion engines are equipped with exhaust gas aftertreatment systems. One of the functions of the aftertreatment systems in case of Diesel engines is the treatment of NOX emission, considering that it is required by current and future legislation, to reach very stringent emission targets in terms of NOX quantity.
Several aftertreatment systems have been proposed to reach such targets, one of which is an aftertreatment system including a Lean NOX Trap upstream of an SCRF. As known, a Lean NOx Trap (LNT) is a device that is used to reduce nitrogen oxides (NO and NO2) and is a catalytic converter support coated with a special washcoat containing zeolites, while a SCRF is a passive SCR (Selective Catalytic Reduction) catalyst coated on a porous DPF (Diesel Particular Filter). Lean NOX Traps (LNT) are subjected to periodic regenerations to release and reduce the trapped nitrogen oxides (NOX) from the LNT.
In order to perform a regeneration event, also referenced as a DeNOX regeneration, Lean NOX Traps (LNT) are operated cyclically, for example by switching the engine from a lean burn operation to a rich operation. Known LNT control strategies are designed to manage DeNOX regenerations of the LNT as a function of air-to-fuel ratios in the exhaust gas, also known as lambda. In particular, known strategies request a DeNOX regeneration if the LNT NOX conversion efficiency is lower than a predefined threshold thereof, namely if the NOX quantity stored in the LNT is greater than a predefined threshold. Once requested, a DeNOX regeneration is ended when a lambda signal breakthrough is verified, namely at the instant in which an air-to-fuel ratio at an LNT outlet is lower than an air-to-fuel ratio at an LNT inlet.
It is also known that the LNT has the capability of producing ammonia (NH3) during a regeneration event. Ammonia is used by the SCRF downstream of the LNT to improve the total NOX conversion efficiency.